In the case of converting voice signals into PCM signals for use in communication, there are two types of encoding and decoding laws (A-law, .mu.-law) recommended by the International Telegraph and Telephone Consultative Committee (CCITT).
The A-law uses a conversion characteristic for encoding as shown in FIG. 1 and a conversion characteristic for decoding as shown in FIG. 2. Accordingly, the total conversion characteristic for encoding and decoding becomes as shown in FIG. 3. That is, it has smaller quantizing steps for smaller absolute values of signal amplitudes and larger quantizing steps for larger absolute values. Here, VF.sub.X is a maximum encoding amplitude, and VF.sub.R is a maximum decoding amplitude of FIG. 3 in which analog inputs X.sub.n are close to 0 (zero). However, digital codes shown in the above diagrams are intended to simplify the explanation and are not A-law output character signals (PCM OUTPUT in FIG. 7) as will be described later. As apparent from FIG. 4, encoder input levels X.sub.n and decoder output levels Y.sub.n have the relationship of ##EQU1## At the point of input level X.sub.0 =0, accordingly, the output level changes stepwise. That is, whenever the input signal has a level between X.sub.1 and -X.sub.1, the output signal becomes a signal level of either Y.sub.1 or -Y.sub.1.
Such conversion characteristic intends to minimize the quantization noise of signals obtained by encoding analog signals with continuous level values and decoding the encoded signals. On the other hand, in a case where only noise enters an encoder in the idle channel state or in the absence of any voice signal, and no matter how much the amplitude of the noise is suppressed, a noise signal having an amplitude level equal to one-half of the quantizing step level of the encoder, namely, an amplitude level of .+-.Y.sub.1, is involved in the output of a decoder. This phenomenon can be termed the "amplification possessed by the conversion characteristic for encoding and decoding".
On account of the amplfication possessed by the conversion characteristic for encoding and decoding; accordingly, it has been impossible to suppress the idle channel noise characteristic to below a certain value. This is stated in detail by R. H. Shennum et al, "Performance Limitations of a Practical PCM Terminal", The Bell System Technical Journal, January 1962, pp. 156-158.